Live
fish in bowls are great for centerpieces. Make sure that you
buy extra in case some die. These are also great to give as
gifts for guests as parting gifts. Check with the parents
first about giving the children fish. You might consider also
providing small bowls and fish food with them. You could also
allow your guests to catch their own fish by picking them
out of a kiddie swimming pool.

Ask
around for those that might have boogie boards or sand boards
to use as simple decorations.

If
you want to create a neat bubble effect with helium balloons,
tie them to thread approximately a balloon and a half length
apart from each other (closer, if you desire). Depending on
the height you are going for will determine how many balloons
you will tie to each thread. Weight one end of the thread
down with something heavy. The balloons floating up will look
like bubbles (providing you use light blues and whites.) Experiment
with different sizes of balloons, heights of strings and numbers
of balloons on a string to achieve the look you want in the
area that you are working with.

Hang
fishing line from the ceiling or beams with large, construction
paper hooks attached to the bottom. If you are clever, you
could also turn this into a game (putting paper clips on each
of the hooks a child has a certain amount of time to
attach as many fish as the can to as many hooks as they can.)

Take
a large pink balloon and attach eight long pink crepe paper
streamers to all sides of the balloon (spreading them out
evenly.) Attach them to about the middle height of the balloon.
With a black marker, draw a pair of eyes on the balloon and
you have a great looking octopus. You can either let the tentacles
dangle down or you can take each one and fasten them to different
parts of the ceiling. If you have the patience (or employ
the help from some children) you can draw circles on the streamers
to simulate suction cups.

For
fun seaweed, drape green and blue crepe paper from helium
balloons (for an underwater effect.) Make sure, though, that
you have purchased large enough balloons to hold your crepe
paper (you might need to experiment with the lengths of the
crepe paper for this.)